Mitigating hydrogen embrittlement of advanced high-strength steel by controlling carbides (cementite and alloy carbides) and microstructural modification

This work aims to demonstrate that the resistance to hydrogen embrittlement of conventional advanced high-strength steel (AHSS) with a bainitic ferrite structure can be significantly enhanced through two metallurgical strategies. Firstly, a smaller fraction of cementite in a mixture of bainitic ferrite and lath-martensite structures is realized by an intercritical annealing process with rapid cooling. Secondly, a higher fraction of alloy carbides (MC) with a mean size of less than 30 nm is formed through the combined addition of Ti, Mo, and V as micro-alloying elements. The reduction of cementite precipitation in the matrix leads to an increased resistance to hydrogen evolution/adsorption on the surface under cathodic polarization. Moreover, a large number of V-bearing nanoprecipitates ((Ti,Mo,V)-carbides and V-carbides) provides intermediate trapping for hydrogen atoms, effectively delaying the diffusion kinetics of hydrogen towards potential crack-forming areas in the matrix. Consequently, the lower strain reduction during slow strain rate test and the higher resistance to cracking during the four-point bending test in acidic aqueous solutions are manifested in the newly designed AHSS.